The invention concerns a process for the manufacture of impact resistant modified polymers by polymerization, especially by radical polymerization of vinyl-aromatic monomers and ethylene unsaturated nitrile monomers in the presence of soluble rubber.
Impact resistant modified polymers manufactured by radical polymerization of aromatic monomers and ethylene unsaturated nitrile monomers in the presence of rubber are known under the name "ABS polymers" or "ABS molding material" (acrylonitrile-butadiene-styrene). Another type of impact resistant modified polymers, known as "HI-PS" polymers (high impact polystyrene), are obtained by polymerization of vinyl-aromatic monomer in the presence of rubber.
Advantageous for ABS manufactured by solution or mass polymerization is the higher rubber efficiency and the avoidance of wastewater, as well as the smaller usage of pigments due to the lighter natural color compared to ABS manufactured by the emulsion process. Disadvantageous for ABS manufactured in mass or solution polymerization is the lack of surface luster (herein "gloss") compared to ABS manufactured by emulsion polymerization. The lower gloss is the result of the relatively large, dispersed rubber particles. There was, therefore, no shortage of attempts to eliminate this disadvantage and manufacture lustrous/glossy ABS in mass or solution polymerization.
A continuous process for the manufacture of ABS polymerizates is described in DE 4 030 352 in which the phase inversion takes place in a Ringspalt reactor, in which the rubber phase passes over from the outer phase to the inner separated phase, and accordingly the polystyrene co-acrylonitrile phase from the inner phase to the outer connected phase. A disadvantage is that at least three reactors are needed for continuous polymerization, and that the shearing stress prevailing in the Ringspalt reactor is relatively small.
Continuous processes for the manufacture of presently relevant resin were described in JP 0408020 and in U.S. Pat. No. 5,210,132 (corresponding to EP-A376 232). In the process disclosed in the '020 document, the reaction solution is sheared by the use of a particle dispenser having wings or rotors rotating at high and low speeds alternatively. The dispersion of the rubber particles is said to be controlled by the speed of the rotation. The process disclosed in the '132 document refers to shear rates which are preferably equal to or greater than 300 s.sup.-1 The application of shear is by a particle disperser having one shearing stirrer composed of rotatable blade or cylindrical rotor rotating at a high speed. The maximum shear rate demonstrated in the example (Example 33) is less than 3000 1/s and there is no indication at all of the criticality of shear rate to the reduction in particle size or the width of the particle size distribution. In fact, the document in column 12, lines 27-39, relates the distribution of the particles to the reaction conditions. Moreover, the working and comparative examples provide no suggestion respecting the present invention which resides, in part, in the finding of critical dependence of both the reduction in particle size and narrowing of the particle size distribution on the shear rate. Also presently relevant are U.S. Pat. Nos. 5,514,750 and 5,550,186 which disclosed the application of shear in relevant processes. Shear rates in the range of 2-2000 1/s were disclosed and higher shear rates were taught away from (see col. 10, line 43 in the '750 patent and col. 11, line 53 in the '186 patent). The disadvantage of the prior art processes is the energy inefficient operation of the rotor/stator/machinery, which leads first to heating of the reaction material, and only second to the breaking up of the rubber particles.
The inventive process relates to a continuous manufacture of impact resistant modified polymers having increased gloss and improved impact properties. The process comprises polymerization, preferably free-radical polymerization, of vinyl-aromatic monomers with or without ethylene unsaturated nitrile comonomers, in the presence of a soluble rubber and optionally in the presence of solvents. In the process, upon completion of the phase inversion, at least part of the reaction mixture is sheared at a rate of at least 30,000, preferably 35,000 to 20,000,0001/s using a device which entails no rotating parts. In view of the state of the art, it was surprising that such high shear rates do not lead to a breakdown of the phases, and that the process may be carried out in the presence of a solvent. It is also surprising that the process achieves higher gloss of the impact resistant modified polymers obtained. It is also surprising that a reaction mixture containing polymerizable mixture and potentially gel forming and easily crosslinkable rubbers can be subjected to high shear rates without formation of gel particles, hard spots or pluggage of the dispersing devices.
The process is preferably carried out in two or more reactors arranged in sequence. Stirred tank reactors with or without recycle loop, tower reactors or plug flow reactors, may be used and they may be filled or partially filled. Preferred are the homogeneous agitated reactors and plug flow reactors. In the case where two reactors are used, the monomer conversion in the first reactor is already sufficiently high that the first reactor is past the phase inversion, i.e., that rubber particles exist in a predispersed form. In cases where three or more reactors are used, it is possible to operate all three reactors after the phase inversion, or, the first reactor before, and the second and third reactors after the phase inversion. The inventive process is preferably carried out in two or three stirred tank reactors. In a specially preferred embodiment, the process is carried out in two such reactors.
The high shear rates may be generated by pumping the reaction mixture through static dispersing devices, containing no rotating parts, at high pressures. A static mixer may be used as a dispersing device. Common to all static mixers is that a liquid flow in a tube is constantly separated, relocated, combined, and redistributed by internal components. The pressure energy available is thereby dissipated in small volumes.
Also, a jet dispergator may be used as a dispersing device in which the pressure energy is dissipated in small volumes in a pressure relief nozzle. Other suitable static dispersion devices include microporous filters, microporous glass filters microfluidizers and Manton-Gaulin homogenizer nozzles. The jet dispergator is the preferred device.
A critical aspect of the invention therefore resides in that the weight average particle size of the rubber (herein "d.sub.w ") decreases, and the width of the size distribution of the particles (herein "d.sub.w /d.sub.n ", where d.sub.n denotes the number average particle size) narrows by shearing in accordance with the inventive process. In accordance with the inventive process, the application of shear at the inventive rate reduces dw and d.sub.w /d.sub.n by at least 20%, preferably by at least 30% relative to the values obtained by the process but for the application of shear stress at the prescribed rate. This application of shear stress is upon the completion of the phase inversion. A jet dispergator contains a pressure relief nozzle in which the available pressure energy is dissipated in the smallest possible volume in a dispersion zone, and with this a high volume specific dispersion power is achieved. Suitable design types are described in DE 195 10 651 (FIGS. 1, 2 and 6) and in EP 101 007 A2 (FIGS. 2, 3 and 4).
Suitable static mixers include the ones available from Sulzer company, Winterthur, Switzerland/Germany (Commercial identification SMX). Common to all static mixers is that liquid flow in a tube is constantly distributed, relocated, combined and re-distributed by internal components. The static mixers are thereby operated in a way similar to the jet dispergator, i.e., the available pressure energy is dissipated in the static mixer.
The volumetric flow at which the polymer solution which contains rubber particles is transported through the shearing equipment is so high that a pressure drop of 2 to 500 bar, preferably between 2 and 50 bar develops. The operating pressure of 2 to 50 bars can be generated with relatively simple and inexpensive pumps. The shear rate (speed gradient, shearing speed) in the shearing equipment is high: at least 30,000 s.sup.-1, preferably 35,000 to 20,000,000 s.sup.-1 most preferably 35,000 to 1,500,000 s.sup.-1. This shear rate is set by the pressure drop. The advantage of jet dispergators lies in the fact that the energy density is very high: a value of 10.sup.7 W/cm.sup.3 is obtained at a 10 bar pressure drop for the jet dispergator, a value of 10.sup.8 W/cm.sup.3 at 50 bar. In comparison, the energy density with rotor/stator machinery is considerably smaller: 10.sup.4 -10.sup.5 W/cm.sup.3. The application of shear in accordance with the inventive process may be carried out by any device through which the rubber particle containing polymer solution may be pumped, and where high shear rates and/or high volume specific dispersion power is attainable. The invention is therefore not restricted to the use of the preferred jet dispergators and static mixers.
The shearing is carried out after the phase inversion. The monomer conversion at which the phase inversion takes place, i.e., the stage at which the rubber phase inverts from the continuous outer phase to the inner separated phase, and rubber particles develop thereby, depends on the rubber concentration, monomer composition, and the concentration, type and quantity of the solvent.
The inventive process is directed to continuous manufacture of ABS or HI-PS polymers which are suitable as thermoplastic molding resins yielding articles having high gloss. The process comprises polymerization of vinyl-aromatic monomers with or without ethylene unsaturated nitrile monomers, in the presence of soluble rubber, and may be carried out in the presence of solvents, preferably in at least two reactors arranged in sequence. The application of high shear after phase inversion leads to a reduction of the particle size of the rubber and a narrowing of the width of the particle size distribution. The shearing is preferably carried out by jet dispergator or by a static mixer. In a corresponding process where no shearing stress is applied the resulting polymers have coarser rubber particles and exhibit lower gloss values.
In an additional embodiment of the invention, products having a bimodal distribution of particle size are prepared by subjecting only part of the reaction mixture to-the shear. These products are characterized by their improved impact properties.
The shearing equipment to be used in the inventive process in reducing the particle size of the rubber may be installed in a re-circulation loop of the reactor which is the first one following the phase inversion, advantageously a stirred tank reactor. Altematively, the shearing equipment is installed between two reactors. Preferred in this case is that the shearing equipment be installed between the reactor in which the phase inversion takes place and the reactor following it.
During polymerization, a solution of the rubber in the monomers, and optionally solvents, is metered continuously into the reactors that are arranged in sequence. The polymerization solution from the first reactor is continuously fed into the following reactor. If more than two reactors are used, the operation is repeated in the same way. The monomer conversion in the last reactor of the cascade is 30 to 95%, the solid content is 30 to 90 weight %. The polymerization is advantageously initiated by a radical-forming initiator but may also be thermally-initiated; the molecular weight of the polymer formed may be adjusted by the use of well known chain transfer agents. Residual monomers and solvents may be removed by conventional methods (for example, in heat exchanger evaporators, falling film evaporators, extrusion evaporators, thin film or thin layer evaporators, screw evaporators, agitated multi-phase evaporators with kneading and scraping devices), and stripping agents, for example steam, may be incorporated in the inventive process. During the polymerization and the polymer isolation steps, additives, stabilizers, antioxidants, fillers, lubricants and colorants may be added.
Suitable vinyl-aromatic monomers include styrene, .alpha.-methyl styrene, which may optionally be alkyl-substituted or chlorine-substituted. Styrene and .alpha.-methyl styrene are preferred. The suitable ethylene unsaturated nitrile monomers include acrylonitrile and methacrylonitrile.
Additional monomers may be included in the reaction mixture in an amount of up to 20% of the total weight of its monomers. These include acrylic monomers (for example, methyl(meth)acrylate, ethyl(meth)acrylate, tert-butyl-(meth)acrylate, n-butyl(meth)acrylate) maleic acid derivatives (for example, maleic anhydride, maleic acid ester, n-substituted maleinimide) acrylic acid, methacrylic acid, fumaric acid, itaconic acid, and the corresponding amides and esters (for example, butylacrylate and dibutyl-fumarate). Examples of n-substituted maleinimides are n-cyclohexyl, n-phenyl, n-alkyl-phenyl-maleinimide.
The weight ratio of vinyl-aromatic monomers to ethylene unsaturated nitrile monomers is, for ABS manufacture, 60-90/40-10.
Suitable rubbers are soluble in the reaction mixture at the process temperature and include polybutadiene, styrene-butadiene copolymers in statistical and/or block form, acrylonitrile-butadiene copolymers, chloroprene rubbers, and ethylene-propylene rubbers. The solution viscosity of such soluble rubbers, determined on their solution (5 weight %) in styrene is 10 to 200 mPa.multidot.s. The molding resin (ABS or HIPS) manufactured based on the inventive process contains rubber in an amount of 5 to 35% relative to the weight of the resin.
Solvent suitable in carrying out the polymerization in one relevant embodiment of the invention are aromatic hydrocarbons such as toluene, ethylbenzene, xylenes and ketones such as acetone, methylethylketone, methylpropylketone, methylbutylketone, as well as mixtures of these solvents. Preferred are ethylbenzene, methylethylketone and acetone, as well as their mixtures.
The average polymerization process residence time is 1 to 10 hours. The polymerization temperature is 50 to 180.degree. C.
Initiators for radical polymerization are well known. Examples include azodiisobutyric acid dinitrile, azoisobutyric acid alkylester, tert-butylperpivalate, tert-butylperoctoate, tert-butylperbenzoate, tert-butylpemeodekanoate, tert-butylper-(2-ethylhexyl) carbonate.
These initiators may be used in quantities of 0.005 to 1 weight % in reference to the monomers.
In order to adjust the molecular weights, conventional chain transfer agents may be used in amounts of 0.05 to 2 weight % relative to the reaction mixture. These include mercaptans and olefins, for example, tert-dodecylmercaptan, n-dodecylmercaptan, cyclohexene, terpinolene, and .alpha.-methylstyrene dimers.
The products obtained based on the inventive process have rubber particle sizes (weight average d.sub.w) of 0.1-10 .mu.m, preferably 0.1-1 .mu.m. The products based on the inventive process preferably show a melt index of 1-60 (220.degree. C./10 kg) [ml/10 min].
The molding material based on the inventive process may be processed thermoplastically, including the known methods of injection molding, extrusion, spray molding, calendering, blow molding, pressing and sintering.
The advantage of the inventive process is that with the application of high shearing forces, using the high shearing forces of a jet dispergator or static mixer, the rubber particle size of HI-PS or ABS may be reduced resulting in lusterous, that is glossy, products. Without shearing, the products contain coarse particles and exhibit lower gloss, and insufficient impact toughness.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.